The invention lies in the field of telecommunication cables having large numbers of optical fibres. More in particular, it relates to a cable having optical-fibre ribbons according to the preamble of claim 1.
Such cables are known per se, e.g., from references [1], [2] and [3]. With the cables described herein, there is included a tape-shaped stack of fibre ribbons in a tubular cavity located centrally within the cable sheath and having a substantially rectangular cross section. Strength elements applied on either side of the tubular cavity determine a preferred bending plane corresponding to a preferred bending plane of the stack located in the tubular cavity. The stack in the tubular cavity extends in the longitudinal direction of the cable.
In the type of cable of reference [1], the fibre ribbons are located loosely, one on top of the other, in the stack in the tubular cavity, which is preferably filled with a water-blocking material. In the type of cable of reference [2], the tape-shaped stack extends as a waveform into the tubular cavity. Said waveform on the one hand guarantees a surplus length to enable dealing with tensile forces and temperature fluctuations, and also rendering the reeling of the cable possible. On the other hand, such waveform may prevent tilting and/or turning over of the stack, while the fibre ribbons still lie sufficiently loosely in the tubular cavity. With said known type of cable, the remaining space in the tubular cavity may be filled up with a (very weak) filler as well. The type of cable known from reference [3] comprises a strength element which, due to its shape, determines the preferred bending plane, and in which the tubular cavity having the rectangular cross section is located. Said known types of cable have the restriction that the space available in the tubular cavity is utilised, or may be utilised, insufficiently for including as large a number of fibre ribbons as possible. Moreover, said known types of cable have the restriction that they are manufactured with a specific number of fibres, and that they do not offer the option, therefore, of adjusting the number of fibres per cable to a changing need after manufacture or installation.
According to a first aspect of the invention, there is provided for a cable of the aforementioned type which, inter alia, embodies the option of dealing with the restrictions indicated above and relating to the known cables. For this purpose, the invention provides for a cable having optical-fiber ribbons of the above type, characterized as in claim 1. The dimensions of the tape-shaped spacer are such that the stack, including the fiber ribbons, can hardly turn over, or turn around. Moreover, it is possible to modify the number of fiber ribbons in the stack. In a preferred embodiment of the invention, the cable is characterized as in claim 2. As a result of such a transverse convexity of one or both sidewalls of the tubular cavity, which are parallel to the preferred bending plane of the cable body, it is achieved on the one hand that there is sufficient space for a fluid (gas or liquid) flushing through under pressure in order to be able to impel the spacer through the tubular cavity by the drag forces of such a flowing medium. On the other hand, said transverse convexity offers the option of a slight turning over of the stack and of a slight transverse bending of the individual fiber ribbons in the plane of the fiber ribbons in the stack, in the event that the cable body is subjected to curves and/or torsions, which do not correspond sufficiently to the preferred bending plane of the cable body. Such an option also promotes the capability of the cable to be reeled.
In a further preferred embodiment of the cable, the tape-shaped spacer is built up of a number of mutually separable layers. One or more of the mutually separable layers preferably consist of dummy fiber ribbons.
According to a second aspect of the invention, the object is to provide a method for modifying the number of fiber ribbons in a cable according to the invention. To this end, the invention provides for a method according to claims 9 for modifying, i.e. increasing or reducing, the number of fiber ribbons in such a cable.
According to a third aspect of the invention, the object is to provide a method for reeling a cable according to the invention. For this purpose, the invention provides for a method according to claim 14. Such a reeling method considerably reduces the extent to which undesirable tensions may occur in the optical fibers. In this case, particularly the aforementioned transverse convexity is advantageous.
In reference [4], there is disclosed a cable having a cable body provided with slot having a more or less rectangular cross section, in which there are loosely included an optical-fiber ribbon and a non-optical tape-shaped element having a surplus length evenly distributed in a undulatory pattern. The slot is further filled up with a water-blocking gel. The non-optical tape-shaped element serves to preserve the undulatory pattern of the fiber ribbon as well as possible.
In reference [5], there is disclosed a optical ribbon cable comprising a large number of cable veins. The cable veins have a body with a tubular cavity with a mainly circular cross section containing a stack of optical fiber ribbons sandwiched between two band-like fill-elements and embedded in a synthetic material in a fixed position.
The cable according to the invention may have been manufactured with a first number of fiber ribbons and a spacer, whereafter, in the event of a changed need, the number of fiber ribbons in situ may be adjusted in the installed cable. The cable body may also first be installed empty, e.g., in a duct or directly in the soil, using a conventional installation technique. Subsequently, a stack initially having a desired number of fiber ribbons and the spacer are introduced. Together with a spacer, the stack has, at least approximately, the height of the largest stack which may be introduced into the tubular cavity of the cable body. Said introduction is also effected using introductory techniques known per se, such as by blowing in using an air flow, or by flowing in using a liquid. For as long as a spacer is still present (i.e., the thickness of the spacer is greater than zero), the number of fiber ribbons in the stack may still be increased. This is done by removing the spacer present, and introducing one or more fiber ribbons to be added, whether or not together with a thinner spacer, into the vacated space. In this manner, the cable may be gradually completed at the installation site. Conversely, fiber ribbons may also be removed while applying a thicker spacer.
[1] GB-A-2215081;
[2] WO-A-91/00536;
[3] U.S. Pat. No. 5,319,730;
[4] EP-A-0407076;
[5] EP-A-0602447.
The references are considered to be incorporated in the present application.